An ophthalmoscope is an instrument for examining the fundus oculi or concave interior of the eye, particularly the retina, optic disk, and visible blood vessels. Ophthalmoscopes are also used for examination of the vitreous body, the clear jelly that fills the eyeball posterior to the lens. Ophthalmoscopes may be monocular or binocular, the latter providing a stereoscopic view of the fundus, and generally are categorized as direct ophthalmoscopes or indirect ophthalmoscopes.
The direct ophthalmoscope is typically a hand-held instrument which illuminates and magnifies the fundus. Since the fundus is viewed directly, the image seen is erect rather than inverted. Typically, however, the practitioner must position his or her eye in close proximity to the instrument and patient. Moreover, the direct image has a narrow field of view and lacks depth because it is focused with only one of the examining practitioner's own eyes. For these reasons, the direct ophthalmoscope is generally unsuitable for surgical use.
In indirect ophthalmoscopes, a magnifying lens, known as a condensing lens, is interposed between the subject eye and the observer. Thereby, an image of the fundus is created at an image plane located between the patient and the viewer. This is known as indirect ophthalmoscopy, because the image which is viewed by the observer is the image created by the condensing lens. Typically, the condensing lens is used in conjunction with a binocular observation device which reduces the practitioner's interpupillary distance, allowing for stereoscopic viewing of the fundus image. This observation device also typically provides illumination of the fundus under observation.
The aerial image of the retina or vitreous produced by the condensing lens of the indirect ophthalmoscope is inverted and flipped spatially (i.e., left-to-right) with respect to the true original subject. The true retina-vitreous relationships cannot, therefore, be perceived directly. They can only be recreated after further processing, such as by drawing or photography. This creates a problem for the observer, and particularly the eye surgeon who must conduct precise operations in real space on the subject eye to achieve the goals of retinal-vitreous diagnosis and treatment. Since the surgeon's image is inverted in the vertical and flipped in the horizontal plane, his or her manipulations on the eye seem flipped and inverted. Thus, manipulations on the eye are made hazardous.
The condensing lens also reverses the relationship between left and right images of the original object. This becomes a problem when a true oriented stereoscopic image of the eye structure under observation is to be created.
Devices have been presented in the prior art which create a properly oriented fundus image using indirect ophthalmoscopy. These include the device described in U.S. Pat. No. 3,475,082 to Strietzel which describes a stereo ophthalmoscope which produces a correctly oriented stereo image using a separating prism to divide image rays into left and right images and reflecting surfaces or prisms to direct the images to eyepieces. A reversing lens system then forms a properly oriented image. U.S. Pat. No. 4,015,898 to Schirmer describes a wide angle stereo ophthalmoscope which produces an upright, unreversed stereo image using a single Uppendahl prism which inverts, reverses, and separates the rays passing therethrough from the condensing lens. U.S. Pat. No. 4,248,505 to Muchel, et al., describes a stereo ophthalmoscope using a prism and lens system for splitting and reversing the indirect image. U.S. Pat. Nos. 4,699,480 and 4,710,002 to Pomerantzeff describe magnifying stereo ophthalmoscopes using mirrors for splitting and transiently reversing the indirect image. U.S. Pat. No. 4,710,000 to Spitznas, et al. describes a stereo surgical microscope which uses reflecting prisms or mirrors for reversing an image. U.S. Pat. No. 4,786,161 to Muller, et al. describes a stereo ophthalmoscope device using two sets of four identical Porro prisms for image interchange and inversion. U.S. Pat. No. 4,838,678 to Hubertus describes a magnifying binocular ophthalmoscope using Schmidt or Pechan prisms to invert an image.
Use of many of these devices, however, is made difficult by their size and the requirement of a short working distance from the subject eye. Working distance is a particularly important consideration in operating room environments, where contamination of the sterile working field is a continuous concern, and where unnecessary restrictions on the surgeon's work space are to be avoided. In particular, prior art indirect ophthalmoscopes which present a properly oriented image often involve the use of a contact lens on the subject eye. These contact lenses are wide-field lenses, which, due to the large field angle, give reduced resolution of details of the fundus. Moreover, the contact lens can interfere with surgical activities. Devices adapted to operating microscopes have also been created which re-orient indirect images of the retina-vitreous. However, these devices are not portable.
Known ophthalmoscopes use a combination of lenses and complex prisms, in addition to mirrors, to reorient the indirect image of the eye structures of interest. However, the use of lenses in such systems introduce chromatic and spherical aberrations into the images. The use of complex prisms can further introduce image distortion. Finally, image brightness is lost with each optical manipulation.
There are several examples of ophthalmoscopes in the prior art which re-orient an inverted image. It is the object of this invention, however, to provide an optical system to reorient the indirect ophthalmoscopic image while maintaining stereopsis and image clarity without the aberrations attendant to prior art types of optical manipulation.